To constrain the basic features of the sources of UHECR it is useful to adopt a purely phenomenological approach in which sources are homogeneously and isotropically distributed, characterised by few parameters: the injection of particles by the sources with a power law behaviour in energy \(\propto E^{-\gamma _g}\) , the maximum acceleration energy \(E_{max}\)  and the source emissivity \({\cal L}_S(A)\)  , i.e. number of particles injected per unit volume and energy for each nuclei species labeled by the atomic mass number \(A\) . These parameters, that univocally identify a class of sources, are fitted to experimental data of both spectrum and mass composition (see [1]}, [2]}, [3]}, [4]} and references therein).
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